The purpose of this project is to carefully document declining function in the compartments of the rat immunologic system as measured by several standard indices. Particular attention has been directed to comparing blood lymphocyte proliferative responses to selected mitogens with lymphocytes found in the spleen and thymus, since blood is a tissue that can be easily and repeatedly sampled over time from the same subject animal. Presently the work is in progress and has demonstrated a marked fall off of function as measured by thymidine uptake in old rats in comparison to young animals.